1. Field of the Invention
The present invention relates to an optical bidirectional transmission system and method for bidirectionally transmitting an optical signal either between different terminal stations or between a terminal station and a repeater station using a new synchronous transmission method which will now be described. The new synchronous transmission method is a transmission method such as that for bidirectionally transmitting optical signals that uses a hierarchy of data sets, each data set being transferred at a predetermined reference data transfer rate and is standardized as the SDH (Synchronous Digital Hierarchy) or the SONET (Synchronous Optical Network). See a standard No.TR-NWT-000253 for the SONET and CCITT G.707, 708, and 709 for the SDH. The present invention particularly relates to a provision to be provided for such an optical bidirectional transmission system and method, the provision properly detecting a trouble of transmission-line disconnection in the system and method.
The SDH has been determined by CCITT (International Telegraph and Telephone Consultative Committee, now being an ITU-TS (International Telecommunication Union-Telecommunication Standardization Section)). In the SDH, a final data transfer rate is obtained from multiplying an integer by a predetermined reference data transfer rate of 155.52 Mbps (referred to as STM-1). Currently, a data transfer rate of 622.08 Mbps (STM-4) obtained from multiplying the STM-1 by 4 and a data transfer rate of 2.48832 Gbps (STM-16) obtained from multiplying the STM-1 by 16 have been standardized.
The above-mentioned SONET uses a basic rate of 51.84 Mbps (OC-1) and has standardized a data transfer rate of 155.52 Mbps (OC-3) obtained from multiplying the OC-1 by 3, data transfer rate OC-12 obtained from multiplying the OC-3 by 4 and data transfer rate OC-48 obtained from multiplying the OC-3 by 16. Thus, the data transfer rates OC-3, OC-12 and OC-48 standardized in the SONET correspond to the data transfer rates STM-1, STM-4 and STM-12 standardized in the above SDH.
2. Prior Art
Various optical transmission methods have been proposed and made practicable, the optical transmission methods using optical signals obtained from converting electric signals thereto and using optical fibers serving as optical transmission lines. Either a method or a system connecting optical multiplexer-demultiplexers to an optical transmission line so as to bidirectionally transmit optical signals via the optical transmission line has been known. Such optical transmission methods are applied to the above new synchronous transmission method as described above and the bidirectional optical signal transmission can be performed in the new synchronous transmission method.
Further, data multiplexing is performed in the new synchronous transmission method as follows: 24 channels, each channel consisting of 64 kbps of information (corresponding to one channel in a standard telephone data transfer rate), for example, are multiplexed so as to obtain 1.5 Mbps of information. 4 sets of information, each set consisting of the 1.5 Mbps of information, are multiplexed so as to obtain 6 Mbps of information. 7 sets of information, each set consisting of the 6 Mbps of information, are multiplexed so as to obtain 50 Mbps of information. 3 sets of information, each set consisting of 50 Mbps are multiplexed so as to obtain 150 Mbps of information. Thus, the above-described data transfer rate STM-1 (corresponding to 2016 channels in the standard telephone data transfer rate) is obtained. A data frame of the rate STM-1 can be represented by a two-dimensional byte arrangement of 9 rows.times.270 columns. A data frame of a rate STM-0 being 1/3 of the rate STM-1 can be represented by a two-dimensional byte arrangement of 9 rows.times.20 columns, corresponding to a data transfer rate of 51.84 Mbps (corresponding to 672 channels in the standard telephone data transfer rate).
Either a method or a system is assumed in which an optical transmission station and an optical reception station are connected to an optical transmission line via optical multiplexer-demultiplexers, respectively, and the optical transmission line is used to bidirectionally transmit optical signals. In such a method or a system, if a line-disconnection trouble occurs in the optical transmission line, the optical signals are reflected by a part of the transmission line at which the line-disconnection trouble has occurred. The thus reflected optical signals are thus returned to the optical transmission and reception stations, respectively. Since the thus returned optical signals have signal levels similar to those which normally transmitted optical signals have, such a line-disconnection trouble cannot be detected by only monitoring the signal levels of the received optical signals.
Provisions to add an extra bit for detecting line-disconnection troubles in the transmission data frame has been proposed. The provisions will now be described with reference to FIG. 1. An optical transmission system shown in FIG. 1 includes two stations A and B, multiplexers (MUX) 111a and 111b and demultiplexers (DMUX) 112a and 112b being connected to an optical transmission line 114 via optical-electric signal-converting, optical multiplexer-demultiplexers 113a and 113b. Optical signals are bidirectionally transmitted between the stations A and B via the optical transmission line 114. In a case of an example, a setting unit 115a provided in the multiplexer 111a of the station A is assumed to set an extra bit for detecting line-disconnection troubles to be the logical value "0" (low level), while a setting unit 115b provided in the multiplexer 111b of the station B is assumed to set an extra bit for detecting line-disconnection troubles to be the logical value "1" (high level).
An exclusive OR device 116b in the demultiplexer 112b of the station B compares the extra bit for detecting line-disconnection troubles sent from the station A with a reference logical value "0". If no line-disconnection trouble occurs in the transmission line 114, since the extra bit is thus identical to the reference logical value "0", the exclusive OR device 116b provides an output signal having the logical value "0" being provided to an OR device 117b in the demultiplexer 112b. An ER signal is also provided to the OR device 117b and is supplied by an error detecting unit detecting errors which may occur in the station B. The ER signal has the logical value "1" only when the error detecting unit has detected an error in the station B. Thus, if the signal supplied by the exclusive OR device 116b and the signal ER have the logical values "0", the OR device 117b provides an alarm signal ALM having the logical value "0" indicating errors occur neither in the station B nor in the transmission line 114.
Similarly, an exclusive OR device 116a in the emultiplexer 112a of the station B compares the extra bit for detecting line-disconnection troubles sent from the station B with a reference logical value "1". If no line-disconnection trouble occurs in the transmission line 114, since the extra bit is thus identical to the reference logical value "1", the exclusive OR device 116a provides an output signal having the logical value "0" being provided to an OR device 117a in the demultiplexer 112a. An ER signal is also provided to the OR device 117a and is supplied by an error detecting unit detecting errors which may occur in the station A. The ER signal has the logical value "1" only when the error detecting unit has detected an error in the station A. Thus, if the signal supplied by the exclusive OR device 116a and the signal ER have the logical values "0", the OR device 117a provides an alarm signal ALM having the logical value "0" indicating errors occur neither in the station A nor in the transmission line 114.
If a line-disconnection trouble occurs in a middle of the transmission line 114, optical signals including the extra bits for detecting line-disconnection troubles are reflected at the middle of the transmission line 114 as indicated by broken lines in FIG. 1. Thus, the extra bit for detecting line-disconnection troubles of "0" sent from the station A is supplied to the exclusive OR device 116a in the demultiplexer 112a and the device 116a compares the thus supplied extra bit of "0" with the reference logical level "1". The device 116a therefore outputs a signal of "1" and the OR device 117a outputs the alarm signal ALM of "1". Similarly, the extra bit for detecting line-disconnection troubles of "1" sent from the station B is supplied to the exclusive OR device 116b in the demultiplexer 112b and the device 116a compares the thus supplied extra bit of "1" with the reference logical level "0". The device 116b therefore outputs a signal of "1" and the OR device 117b outputs the alarm signal ALM of "1". Thus, the line-disconnection trouble can be detected.
However, in such a method for detecting line-disconnection troubles, addition of extra bits for detecting line-disconnection troubles added to optical signals degrade transmission efficiency. Further, extra provision is necessary to be made in the multiplexer and demultiplexer of each station for adding the extra bits and extracting the extra bits from a received optical signal, thus increasing costs of the transmission station equipment.